The present invention relates to a method for production of microsilica having a high light reflectivity or whiteness.
Microsilica is normally produced as a by-product during production of ferrosilicon and silicon in electric reduction furnaces where a charge comprising a SiO2 source and one or more solid carbonaceous reduction agents is reacted to form ferrosilicon or silicon. In this process, gaseous SiO is formed as an intermediate product in the reaction zone in the furnace and the gas moves upwards through the charge. A part of the SiO gas is condensed in the cooler charge above the reaction zone, while the remaining part of the SiO gas escapes from the charge, is quickly cooled and oxidized by air which is supplied to the furnace, above the charge, and forms particulate amorphous SiO2. The particulate SiO2 is carried upward out of the furnace in the off-gas and is recovered from the furnace off-gas in filters, normally baghouse filters. Microsilica produced in this way has a particle size substantially between 0.02 and 0.5 microns, and the individual particles are basically of spherical shape. Microsilica has during the last two decades found an increasing use as an additive material in concrete, refractory materials, ceramic materials, oil well cementing slurries, plastic materials, paper and others.
In production of ferrosilicon and silicon by the above mentioned method, the carbonaceous reduction agent normally comprises a mixture of about 65% by weight of coal with the remainder being coke and optionally wood chips. This mixture has been shown to give the best possible furnace operation as to productivity and yield of ferrosilicon and silicon.
Microsilica which is recovered by this method has a reflectivity between 30 and 50 measured by a method where black felt has a reflectivity of zero and where BaSO4 has a reflectivity of 98.6. The produced microsilica thus has a relatively dark color, which is a problem where microsilica is intended for use in a white product. The microsilica has such a low reflectivity because the microsilica particles contain carbon in an amount of up to 3% by weight.
Table 1 shows the chemical composition and some other properties for microsilica produced by the conventional method in a furnace for production of 75% ferrosilicon.
Two methods have been proposed to solve the problem with microsilica having a low reflectivity. In one method, microsilica produced as a by-product in electric smelting furnaces for production of ferrosilicon and silicon is heat treated in a fluidized bed at temperatures of up to 900xc2x0 C. in order to combust the carbon contained in the microsilica. This method is described in Japanese patent publication No. 11559/84. According to the other method, microsilica is produced in a so-called microsilica generator from a charge consisting of SiO2 and Si. In this process a small part of silicon is produced in addition to microsilica. Both of these two methods have disadvantages. Heat treatment of microsilica in a fluidized bed implies an additional step which is very costly and which is difficult to control. Without a strict control of temperature and retention time, in the fluidized bed, a part of the amorphous SiO2 particles will be converted to a crystalline state which will give a product with completely different properties. In addition, crystalline SiO2 constitutes a health risk. Production of microsilica in a microsilica generator is very costly and it is difficult to design microsilica generators having a high capacity.
Thus, there is a need to provide a method for producing microsilica whereby the disadvantages of the prior art methods are overcome.
Accordingly, the present invention relates to a method for producing microsilica having a reflectivity between 65 and 90% in a smelting furnace for production of ferrosilicon or silicon by using a charge comprising an SiO2 source and a solid carbonaceous reduction agent, where microsilica is recovered from the off-gases from the smelting furnace, comprises supplying a solid reduction agent to the furnace which contains an amount of volatile matters of less than 1.25 kg per kg produced microsilica; and keeping the temperature in the gas atmosphere in the furnace above 500xc2x0 C.
This process is intended for use in an industrial size furnace which produces ferrosilicon or silicon on a commercial scale.
Keeping the temperature in the gas atmosphere in the furnace above 500xc2x0 C. means that all of the atmosphere above the charge in the furnace is above 500xc2x0 C. This means that the gas atmosphere at the top of the furnace is above 500xc2x0 C. and that the temperature of the gas atmosphere directly above the charge is also above 500xc2x0 C.
The amount of volatile matters in the solid reduction agent is preferably kept below 1.0 kg per kg produced microsilica, while the temperature in the gas atmosphere in the furnace is preferably above 600xc2x0 C. For best results, the amount of volatile matters in the reducing agents is kept below 0.5 kg per kg produced microsilica.
It has surprisingly been found that the method of the present invention can produce microsilica having a very high reflectivity at the same time as maintaining the yield of ferrosilicon or silicon. Such is extremely important for commercial smelting furnaces.
Microsilica having a very high reflectivity can thus, according to the present invention, be produced by changing the ratio between coke and coal in the reduction agent mixture and by keeping the temperature of the gas atmosphere at the top of the furnace above 500xc2x0 C.
As coal has a substantially higher content of volatile matters than coke, one will in practice reduce the amount of coal and increase the amount of coke in the reduction agent mixture. According to a particularly preferred embodiment the reduction agent consists completely of coke.
As one of skill in the art recognizes, the temperature in the reaction zone around the arc at the tip of the furnace electrode in silicon and ferrosilicon furnaces is around 2000xc2x0 C. Thus the temperature of the gas around the arc is also around 2000xc2x0 C. The gas generated in the reaction zone moves upwards through the furnace charge in a counter-current flow with the furnace charge. The hot gas preheats the furnace charge and is consequently cooled to a much lower temperature before it enters the space above the furnace charge. Typically, the temperature of the gas atmosphere at the top of the furnace is about 400xc2x0 C. and below.
The filters which collect the particulate SiO2 require that the off-gas have a temperature of around 250xc2x0 C. or less, thus, it is conventional to supply air to the gas atmosphere above the charge in the furnace such that the temperature of the off-gas (gas atmosphere) at the top of the furnace is cooled to about 400xc2x0 C. or below. The off-gas will during the transport from the furnace to the baghouse normally cool down to a temperature of not more than 250xc2x0 C.
One suitable means for keeping the temperature of the gas atmosphere in the furnace and especially at the top of the furnace above 500xc2x0 C. in accordance with the present invention is to reduce the amount of air which enters the furnace. By reducing or controlling the in-flow of air to the gas atmosphere in the furnace above the charge, the temperature of the off-gas (gas atmosphere) at the top of the furnace is prevented from cooling to below 500xc2x0 C. before it leaves the furnace. Controlling the amount of air that enters the furnace is done in a conventional manner using conventional equipment.
In order to measure the temperature of the gas atmosphere, the temperature of the gaseous atmosphere at the top of the furnace is suitably monitored by measuring the temperature of the off-gas as it enters the outlet in the hood of the furnace where the off-gas exits the furnace and enters the pipes which transports the off-gas to the baghouse. Alternatively, the temperature of the gaseous atmosphere in the furnace is measured just below the hood itself.
The off-gas must be cooled, prior to the filter, to about 250xc2x0 C. or below. This step can be accomplished in any conventional manner. For example, by increasing the distance travelled by the off-gas in the pipes that connect the furnace to the baghouse, or by inserting a conventional gas cooling apparatus in the off-gas pipe between the furnace and the baghouse.
By the present invention, microsilica is produced having a whiteness of up to 90 at the same time as the other properties of the produced microsilica are not changed and where the costs for production of microsilica is not substantially higher than in production of microsilica using a conventional reduction agent mixture.